Microstructured optical fibers are well known in the art and include fibers having a core surrounded by a cladding having a plurality of cladding features arranged in a background material, typically in a regular array. The features may be voids filled with air, gas or liquid, or they may be solid material having a lower refractive index than the background material. Microstructured optical fibers are also denoted “holey fibers” and “photonic crystal fibers”. The cladding may for example be arranged to have an effective refractive index that is lower than the refractive index of the core and thus permits the guidance of light in the core by a variation of the traditional mechanism of total internal reflection.
The microstructured fiber may be made of silica glass. Other materials may be added to the silica glass in order to alter the refractive index thereof or to provide effects, such as amplification of light, sensitivity, etc.
The center-to-center spacing between the cladding features/holes is defined as the pitch (A). The microstructured fibers are characterized by the size of the core and the ratio of the size of the cladding features to their spacing or pitch (∧). By tailoring the size and pitch of the cladding features, the zero dispersion wavelength (ZDW) of the fiber may be tailored. In microstructured optical fibers it is thus possible to shift the ZDW to shorter wavelengths than the ZDW of the background material, typically silica glass; hereby, the ZDW may be tailored to enable supercontinuum generation from optical pulses pumped by a pump laser of a predetermined wavelength.
When optical pulses propagate through a highly nonlinear fiber, their temporal as well as spectral evolution is affected by a multitude of nonlinear effects as well as by the dispersive properties of the fiber. For sufficiently intense pulses the pulse spectrum broadens to become a supercontinuum light.
WO2009/098519 describes an optical fiber arranged for providing supercontinuum generation down to wavelengths below 400 nm. Page 7, line 29 to page 8, line 1 describes that a microstructured optical fiber 20 of WO2009/098519 comprises a core substantially of 4.7 μm in diameter, a pitch ∧ of substantially 3.7 μm and that the ratio d/∧ is substantially 0.77. Page 8, line 14-22 describes that the fiber 20 is a multimode fiber, and that the supercontinuum reaches wavelengths of up to 2550 nm. The pump wavelength is 1064 nm. As noted on page 8, line 14, the fiber 20 is multimode at the pump wavelength.
In the article “Very low zero-dispersion wavelength predicted for single-mode modified-total-internal-reflection crystal fibre” by Jacobsen et al, Journal of Optics A: Pure Appl. OPT 6 (2004) 604-607 it was found that for microstructured optical fibres with very small core, the zero dispersion wavelength could be below 700 nm while the fiber being single mode at the zero dispersion wavelength by arranging relatively large air holes close to the core and smaller air holes at a distance from the core.